Quantifying shortwave radiative forcing and heating rates of UTLS aerosols in the Asian summer monsoon anticyclone region

Abstract

This study quantifies the shortwave (SW) direct radiative forcing and heating rates associated with the monsoonal aerosol enhancements in the upper troposphere and lower stratosphere (UTLS) over a broad spatial domain (25°N to 37.5°N and 40°E to 95°E) within the Asian summer monsoon anticyclone (ASMA) region. Using satellites and reanalysis data, we examine three dominant aerosol scenarios—sulfate, nitrate, and anthropogenic aerosols—and demonstrate that these enhancements induce a cooling at the top of the atmosphere (TOA), with radiative forcing estimates ranging from −0.13 to −0.19 Wm^-2. Our analysis reveals increased upwelling SW fluxes at the TOA during the monsoon, with forcings reaching −0.2 Wm^-2 relative to winter conditions. While both scattering aerosols (sulfates and nitrates) and absorbing aerosols (anthropogenic) contribute to TOA cooling, the magnitude of cooling from anthropogenic aerosols is comparatively smaller. Within the UTLS column (12–20 km), heating rates peak at 0.04 K day^-1 for anthropogenic aerosols, while those associated with sulfate and nitrate aerosols remain significantly lower. Additionally, the study identifies key atmospheric changes during the monsoon, including elevated water vapor concentrations, reduced temperatures, and deceased ozone levels, both spatially and vertically, across the study domain. These findings underscore the critical role of Asian tropopause aerosols in shaping the regional radiation budget, emphasizing the need for their inclusion in future climate model assessments.